1. Field
The present inventive concepts relate to heat resistant spheroidal graphite cast iron, a method of manufacturing the same, and an engine exhaust system including the same. More particularly, the present inventive concepts relate to heat resistant spheroidal graphite cast iron having a specific composition ratio and excellent high temperature material properties (for example, eutectoid transformation temperature and high temperature tensile strength), a method of manufacturing the same, and an engine exhaust system including the same.
2. Description of the Related Art
Recently, as demands for improved fuel efficiency are increased and environment-related restrictions are becoming strengthened globally, an engine operates with high power, resulting in an increase of a combustion temperature in an exhaust system component. As such, in order to meet continued demands for corresponding to the increased combustion temperature in a heat resistant part, a heat resistant material having an excellent heat resistant property is being developed necessarily.
Heat resistant spheroidal graphite cast iron including silicon (Si) and molybdenum (Mo) is a representative material used for a vehicle exhaust system component which is required to have excellent high temperature properties. In order to increase eutectoid transformation temperature of the heat resistant spheroidal graphite cast iron and precipitate carbide at high temperature to maintain high strength, silicon (Si) content may be increased and a carbide forming element such as molybdenum (Mo) may be added, respectively.
Especially, in order to maintain desired material properties at the combustion temperature of above 800° C. in the exhaust system, a heat resistant property may be required to have more excellent high temperature material property and lower coefficient of thermal expansion than a widely used Si—Mo based heat resistant spheroidal graphite cast iron.
Studies have been conducted to improve a heat resistant property of a material based on the Si—Mo based heat resistant spheroidal graphite cast iron. Conventionally, in order to improve the heat resistant property, a carbide forming element and an additional alloy element was added, however, during alloy design about a selection of alloy element and an addition rate, quantitative reviews on precipitate prediction was insufficient. Thus, a development level of the conventional Si—Mo based heat resistant spheroidal graphite cast iron was low to the extent that high temperature tensile strength at 700° C. was about 70-80 Mpa and high temperature tensile strength at 800° C. was about 45-58 Mpa. In a conventional art, there was a limit to improve high temperature material properties such as high temperature tensile strength, and it was difficult to apply mass production because of matrix structure instability. Because an increase of high temperature tensile strength can greatly affect a product's stability, various researches are required to be conducted in order to improve high temperature material properties.